New restart delay of Canada's Chalk River reactor

* Restart has been again been delayed

* Was shut down in May 2009 by leak

* Few medical isotope alternatives available

VANCOUVER, March 10 (Reuters) - Restart of the Chalk River medical isotope-producing nuclear reactor has been delayed until at least May because of the complexity of ongoing repairs, Atomic Energy of Canada Ltd said on Wednesday.

The aging reactor in eastern Ontario, which supplies a third of the world’s medical isotopes, was shut down in May 2009 by a small heavy water leak and the date of its return to service has been pushed back repeatedly.

The latest start-up date had been April, but AECL said the complex welding required “first-of-a-kind technical solutions” and they needed to make sure the repairs were not damaging the reactor vessel.

“As a result, the current return-to-service is projected to be during the second half of May 2010. Material risk does exist that this schedule will be adjusted,” the government-owned company said in a written statement.

Officials had initially speculated when the more than 50-year-old reactor broke down last year that they could have it running again in three months. The leak did not pose a risk to the general public.

The overall repair process started in December and the repairs and post-repair examinations are about 46 percent complete, AECL said.

The shortage of isotopes has forced doctors around the world to reschedule or cancel medical procedures and sent companies such as Canada's MDS Inc MDS.TOMDZ.N, whose Nordion division depends on Chalk River for the bulk of its isotopes, to scramble for new supply sources.

A medical isotope is a very small quantity of radioactive material used to perform nuclear medicine imaging tests. Isotopes are mixed with different solutions and injected into patients where they give off energy that is read by a special camera.

MDS has estimated it takes a $4 million a month hit to earnings before interest, taxes, depreciation and amortization, or EBITDA, because of the shutdown. (Reporting Allan Dowd, Editing Carol Bishopric)